The protein product of the histone H2A.Z gene is an essential component of eukaryotic chromatin and is likely to have a critical role in the structure-function relationships that facilitate the transcription of genes in chromatin. The primary objectives of this project are to study the organization of this gene and the mechanisms that regulate its expression in the various states of cell growth, and to determine the function of this evolutionarily diverged but highly conserved histone in chromatin. Transcription of the H2A.Z gene is modulated by the binding of protein factors to specific DNA sequence motifs located just upstream from the core promoter of the gene. These factors either enhance or repress the rate of transcription of this gene depending on the state of cell proliferation or differentiation. Extensive mutation analysis of the transcriptional control regions of the human H2A.Z gene has been done and is allowing us to assess the contributions of the individual cis-acting sequence elements and their cognate .factors to the control of transcription of this gene. We have observed that cotransfected HIV TAT gene acts on the H2A.Z gene promoter to markedly reduce reporter gene expression. The mutant H2A.Z promoter-CAT constructs will also allow us to determine through which cis-element and trans-acting factor the HIV TAT protein is acting to down-regulate expression of this gene.